Questions And Answers

My question is about the use of the chairs we have for our assemblers.

We have the standard cloth roll-a-round adjustable type for all the production lines. We also have constant ESD monitors installed at all the ESD workstations. I was asked to check on the chairs that are listed as static controlled with a drag chain. We don’t have any ESD wax or tile on the floors, just a grey painted surface. I know that with conductive casters the chairs won’t generate static like the typical chairs do, but there are some that think you don’t need to wear a Wrist strap if you are sitting in one of these “ESD Safe chairs. I’m having a hard time convincing them otherwise, can you help me out? -Anonymous, Radio Frequency Systems, Phoenix, Arizona

Answer

Anything that comes into contact with another material and is then separated will generate a charge imbalance or static electricity regardless if the material is metal, grounded or insulative. It is the fact that a material can conduct and is tied to a reference potential (ground) that keeps this imbalance at a safe potential relative to the design of the ESD control program, i.e. zero volts. If an ESD chair is on a non-conductive floor, then it will charge and retain it’s charge until discharged via any external conductive connection (human hand, drag chain to a conductive (ESD) floor, metal foot rest hitting grounded metal table stand, etc.) In any case, when handling sensitive ESD device it is still highly recommended to be directly grounded via a wrist strap.

For your ESD Safe chairs, it’s not necessarily the conductive casters that minimize the tribocharging, but the whole system. The material of the seat and back must be electrically dissipative (conductive) and tied to ground via the chair structure. Whether the chair is grounded or not, the large capacitance change on a human body via sitting (increased surface area) can still cause large charge imbalances on the human skin that needs to be handled via a direct and reliable ground path, e.g. a wrist strap.

One way to prove this to someone is to have them sit in a chair (any chair) and not be grounded (i.e., with wrist strap or foot grounder) but connected to a charge plate analyzer. Then have them stand up while watching the large voltage put on the charge plate, even if only for a second or two.

In lieu of a charge plate, as they are standing, if they were to touch a metal plate or surface, they may experience a discharge (one they can feel), if not a high-speed oscilloscope with a loop antenna hooked into the high-impedance input would pick up an EMI from the ESD to visually see.

This just stresses the fact that having a high-integrity ground path for the human skin is extremely important for any good ESD control program